(1) Field of the Invention
The present invention relates to a powder primer composition and to a method of forming a coating film that employs the powder primer composition.
(2) Description of Related Art
Steel and aluminum wheels are commonly used as automobile parts. Such wheels are required to have a high level of corrosion resistance and an excellent level of design, and there are commonly employed wheels made of aluminum (hereinafter referred to as “aluminum wheels”) that have excellent lightweight properties, which for protection and aesthetic quality are usually coated with, for example, a thermosetting powder primer, subsequently heat cured, and then further coated with a thermosetting acrylic resin-based solvent-type coating as a top coat. Aluminum wheels with a sheen design are coated with a thermosetting powder primer composition, heat cured and then coated with a bright thermosetting acrylic resin-based solvent-type coating as a top coat, further heat cured, and processed by cutting. They are then still further coated with an additional thermosetting acrylic resin-based clear coating.
In such coating methods, however, problems occur at the cut sections where the powder coating film remains as burrs or the coating film is lost, creating severe irregularities between the cut surface and the non-cut surface.
In order to solve these problems, there have been proposed methods in which powder coating is carried out in such a manner that the cured coating film that has been clear coated on a cut aluminum wheel with a bright design has a minimum dynamic elastic modulus (E′) of at least 5×107 Pa (for example, Japanese Unexamined Patent Publication No. 7-18207). Methods of applying powder coatings employing resins composed mainly of bisphenol A-type epoxy resins are also known (for example, Japanese Unexamined Patent Publication No. 6-279709). However, although such methods offer improved machinability, the storage stability of the powder coatings has been insufficient.
The present applicant has already proposed a powder primer composition whose essential components are a carboxyl group-containing polyester resin, an epoxy resin, a phosphoric acid-modified epoxy resin and a rust-preventive pigment (for example, Japanese Unexamined Patent Publication No. 2005-162929 and Japanese Unexamined Patent Publication No. 2005-248046). The powder primer composition has excellent storage stability, and can form coating films with superior finished appearance, cutting machinability, corrosion resistance and chipping resistance. Nevertheless, depending on the ratio of the compounding ingredients in the powder primer composition, the crosslink density of the formed coating film is sometimes too high, increasing the internal stress and resulting in reduced corrosion resistance.